Optical scanning apparatus

ABSTRACT

An optical scanning apparatus includes a first optical member for receiving a plurality of light beams with an interval and for causing a first group of beams to emerge with a narrower interval, the first optical member being rotatable to adjust the interval of the beams emergent therefrom; a second optical member for receiving a second light beam and the first group of beams emergent from the first optical member with an interval and for causing a third group of beams to emerge with a narrower interval, the second optical member being rotatable to change the interval between the first group of beams and the second beam; and deflecting means for scanningly deflecting a third group of beams emergent from the second optical member.

This application is a divisional of application Ser. No. 11/122,085,filed date May 5, 2005, now allowed, now U.S. Pat. No. 7,855,817 whichis a divisional of 10/337,376, filed date Jan. 7, 2003, which issued asU.S. Pat. No. 6,947,190 B2.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an optical scanning apparatuspreferably used as an exposing apparatus for an electrophotographicapparatus, in particular, an optical scanning apparatus which is capableof deflecting three or more beams of light in a scanning manner.

Recently, due to the demand for higher productivity, higher printingspeed, and higher image quality, the research regarding an image formingapparatus such as a laser copying machine, a laser beam printer, etc.,has been focused on increasing recording density.

However, the increase in printing speed has resulted in the increase inthe laser modulation frequency, or the like, creating problems; forexample, it has become difficult for a laser driving circuit to keep upwith the laser modulation frequency required for the increased printingspeed.

There are also limitations regarding a light deflecting device such as apolygon mirror in that a polygon mirror is limited in the number of themirroring surfaces it can have, and that there is a limit to therevolution of the motor for rotating the polygon mirror.

There have been proposed various methods for solving the above describedproblems. According to one of such methods, the entirety of the portionof the peripheral surface of a photoconductive member in the exposingstation, that is, the surface to be scanned, is scanned all at once.

In the case of this type of optical scanning apparatus, a plurality ofbeams of light are projected onto each of the deflective surfaces of alight deflecting device, and the plurality of beams of light deflectedby the deflective surface are guided through a focusing optical system,onto the specific areas of the portion of the peripheral surface of thephotoconductive drum in the exposing station, that is, the surface to bescanned, scanning the specific areas of the portion of the surface to bescanned, all at once, to record image formation information on thesurface.

On the other hand, in the case of an optical scanning apparatus used ina laser copying machine also usable as a printer, a laser beam printer,etc., its resolution (recording density), which is expressed by thenumber of dots (picture elements) per unit length, that is, theso-called light beam pitch, must be changed in the primary as well assecondary scanning directions, depending on various factors, forexample, the ambience in which the computer or the like of the opticalscanning apparatus is operated, the software used by the computer or thelike, etc.

When a light source apparatus which emits a single beam of light isemployed as a means for varying the resolution, its resolution can bevaried by varying the relationship between the revolution of the polygonmirror and the velocity of the peripheral surface of the photoconductivedrum, that is, the surface to be scanned, while varying the light beamemission interval in terms of the primary scanning direction, that is,varying the image formation clock frequency.

However, a method for scanning all at once the entirety of the portionof the peripheral surface of a photoconductive drum in the exposingstation by a plurality of beams of light has a problem in that the abovedescribed means for varying the resolution is not alone sufficient forvarying the pitch, in terms of the secondary scanning direction, amongthe plurality of beams of light projected onto the same deflectivesurface.

Thus, a light source apparatus, in accordance with the prior art, whichemits a plurality of beams of light, was structured so that theplurality of beams of light remained fixed in pitch, and its resolutionwas changed by changing the pitch of the plurality of beams of light onthe peripheral surface of the photoconductive drum in terms of thesecondary scanning direction by rotating the entirety of the lightsource apparatus about its optical axis, or by changing the peripheralvelocity of the photoconductive drum at the same time as the pitch ischanged.

Further, there have been proposed various light source apparatusesemploying a plurality of light emitting elements as means forsynthesizing a plurality of beams of light. One of such light sourceapparatuses, which has two light emitting elements, has a singlesynthesizing portion, and employs a polarizing prism as a synthesizingmeans.

If a light source apparatus, such as the above described light sourceapparatuses, employing a polarizing prism as the synthesizing means, hasthree or more light emitting elements, it must be provided with two ormore synthesizing portions. Therefore, the beams of light from two ormore light emitting elements pass through two or more synthesizingportions, making it impossible to employ a polarizing prism in order toutilize the difference in polarization direction. Thus, in order tosolve this problem, a synthesizing prism equipped with a half mirrorcapable of both reflecting and transmitting at a constant ratioregardless of the polarization direction has been employed.

In the case of the above described prior technologies, however, thefollowing problems have occurred.

That is, optical scanning apparatuses based on the technology whichrotated the entirety of a light source apparatus to switch resolution,were too sensitive for proper adjustment.

Further, the demand for higher productivity led to the increase in thenumber of the beams of light, which in turn necessitated three or morelight emitting elements, as well as additional synthesizing portions. Asa result, the light source apparatuses became substantially larger, andalso, unstable in terms of adjustment.

Moreover, the greater the number of synthesizing portions, the lower thehalf-mirror efficiency. Also as described above, a light sourceapparatus, which has a synthesizing means comprising two or moresynthesizing portions having three or more light emitting elements,employs a synthesizing prism equipped with a half mirror. Therefore,such a light source apparatus requires light emitting elements whichemit a larger amount of light.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide an opticalscanning apparatus, the light beam intervals of which are variable.

Another object of the present invention is to provide an opticalscanning apparatus, which is smaller in the amount of the light lossoccurring during the synthesis of a plurality of beams of light.

Another object of the present invention is to provide an opticalscanning apparatus comprising: a first optical member into which aplurality of beams of light are projected, and from which the pluralityof beams of light projected thereinto are projected as a first group ofbeams of light, the intervals of which are narrower than those as theyare projected into the first optical member, said first optical memberbeing enabled to be rotated to adjust the intervals of the plurality ofbeams of light, on the surface to be scanned; and a second opticalmember into which the first group of beams of light projected from thefirst optical member, and a second beam of light, are projected, andfrom which the first group of beams of light and second beam of lightare projected as a third group of beams of light, the intervals of whichare narrower than those as they are projected into the second opticalmember, the second optical member being enabled to be rotated to changethe intervals among the first groups of beams of light and second beamof light.

These and other objects, features, and advantages of the presentinvention will become more apparent upon consideration of the followingdescription of the preferred embodiments of the present invention, takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of the first embodiment of alight source apparatus in accordance with the present invention.

FIG. 2 is a schematic perspective view of the optical scanning apparatusin the first embodiment of a light source apparatus in accordance withthe present invention.

FIG. 3 is a schematic drawing of a typical image forming apparatus.

FIGS. 4A and 4B are drawings depicting the pitch of a plurality of beamsof light on the surface being scanned, in terms of the secondaryscanning direction.

FIGS. 5A and 5B also are drawings depicting the pitch of a plurality ofbeams of light on the surface being scanned, in terms of the secondaryscanning direction.

FIG. 6 is a schematic perspective view of the second embodiment of alight source apparatus in accordance with the present invention.

FIG. 7 is a schematic perspective view of the third embodiment of alight source apparatus in accordance with the present invention.

FIGS. 8A and 8B are drawings depicting the pitch of a plurality of beamsof light on the surface being scanned, in terms of the secondaryscanning direction.

FIG. 9 is a schematic perspective view of the fourth embodiment of alight source apparatus in accordance with the present invention.

FIGS. 10A and 10B are drawings depicting the pitch of the plurality ofbeams of light on the surface being scanned, in terms of the secondaryscanning direction.

FIGS. 11A and 11B are schematic perspective views of portions of a lightsource apparatus. FIG. 11A shows the portion comprising light emittingelements 901 and 902, and a half mirror 931, which synthesizes lightbeams A1 and A2. FIG. 11B shows the portion comprising light emittingelement 901 having two light emitting portions. The portion shown inFIG. 11B can replace the portion shown in FIG. 11A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the appended drawings.

The present invention is characterized in that an optical scanningapparatus, which optically scans all at once the entirety of the surfaceto be scanned, such as the portion of the peripheral surface of aphotoconductive drum in the exposing station, with the use of aplurality of beams of light, comprises a light beam pitch adjustingmeans capable of switching the light beam pitch (physical scanning lineintervals) of the apparatus between two settings, in terms of thesecondary scanning direction, that is, the direction perpendicular tothe primary scanning direction, in order to make it possible for theimage formation data can be recorded at two different resolutions(recording density). The measurements, materials, and configurations ofthe structural components, as well as the positional relationship amongthem, in the following embodiments of the present invention, are to bemodified as necessary, depending on the structures of the apparatuses towhich the present invention is applied, and the conditions under whichthe present invention is applied. In other words, the followingembodiments of the present invention are not intended to limit the scopeof the present invention.

Embodiment 1

FIG. 1 is a perspective view of the first embodiment of a light sourceapparatus in accordance with the present invention, which is a beamsynthesizing portion for synthesizing a plurality of beams of light.FIG. 2 is a perspective view of an optical scanning apparatus. FIG. 3 isa sectional view of an image forming apparatus employing the apparatusesin FIGS. 1 and 2, as exposing apparatuses.

Referring to FIG. 3, an image forming apparatus 50 makes its opticalscanning apparatus 52 emit a plurality of beams of laser light modulatedwith the obtained image formation information, so that the plurality ofbeams of laser light emitted from the optical scanning apparatus 52illuminate the peripheral surface of the photoconductive member 6 in aprocess cartridge 53.

Next, the image forming operation carried out by the image forming meansof the image forming apparatus 50 will be described.

A latent image is formed on the peripheral surface of thephotoconductive drum 6, and is developed by the process cartridge 53,with the use of toner.

Meanwhile, a plurality of sheets of recording medium are conveyed by thecombination of conveying rollers and a separation pad, while beingseparated one by one, and are conveyed further downstream by anothergroup of conveying rollers. As each of the plurality of recording mediumsheets is conveyed, the image realized on the peripheral surface of thephotoconductive drum 6 with the use of toner is transferred onto thesheet by a transferring means 54.

After receiving an unfixed toner image, the recording medium sheet isconveyed further downstream into a fixing means 5, in which the unfixedimage is fixed to the recording medium sheet. Thereafter, the recordingmedium sheet is discharged from the image forming apparatus by a groupof discharge rollers.

FIG. 2 is a schematic perspective view of an optical scanning apparatus52. This drawing does not show the deflection mirror for guiding theplurality of beams of light onto an image bearing member.

In FIG. 2, a referential numeral 1 stands for a light source apparatuswhich has a semiconductor laser, as a light generating means, comprisingone or more light emitting portions, and which projects three or morebeams of light at the same time.

Designated by a referential numeral 3 is a cylindrical lens capable ofrefracting the beams of light at a predetermined angle, only in thesecondary scanning direction.

A light deflecting device 4 is a rotational polygonal mirror, and isrotated by a motor 13 as a means for rotationally driving the lightdeflecting device 4, at a predetermined constant velocity in thedirection indicated by an arrow mark w1 in the drawing. The revolutionof light deflecting device 4 is kept constant at a predeterminedrevolution, by an unshown control system.

Designated by a referential numeral 5 is a focusing means comprising anf-θ lens, etc., which condenses the plurality of beams of lightdeflected by the light deflecting device 4, and focuses them ondifferent points of the peripheral surface of the photoconductive drum6, that is, the points to be exposed.

The photoconductive drum 6 is rotated by an unshown mechanism at apredetermined constant velocity in the direction indicated by an arrowmark w2 in the drawing.

Designated by a referential numeral 7 is a means for detecting both thesynchronization and pitch of the light beams. This detecting means 7 isat a location Lf, which is the starting point of the scanning of theperipheral surface of the photoconductive drum 6 with the beams oflight. It has a deflection mirror 71, and a circuit 72 which detectshorizontal synchronization signals and light beam pitch.

In order to obtain horizontal synchronization signals (BD signals) foradjusting the timing with which the scanning of the peripheral surfaceof the photoconductive drum 6 is started from the starting point, andthe signals representing the pitch, in terms of the secondary scanningdirection, of a plurality of beams of light (L₁-L_(n)) deflected all atonce by each of the deflective surfaces of the light deflecting device 4in a manner to scan the peripheral surface of the photoconductive drum6, the plurality of beams of light deflected by the light deflectingdevice 4 are each partially received by the light beam pitch detectingcircuit 72 via the deflection mirror 71.

Further, the timing with which the plurality of beams of light areemitted from the light source apparatus 1 in synchronism with the BDsignals from the light beam pitch detecting circuit 72 is controlled byan unshown light emission control circuit, with the use of the BDsignals. Also, the light beam pitch is adjusted by the unshown lightemission control circuit, with the use of the light beam pitch signalsfrom the detection circuit 72.

FIG. 1 is a schematic perspective view of the light source apparatus 1,which comprises four light emitting elements, each of which has a singlelight emitting portion.

Designated by referential numerals 101-104 are semiconductor lasers, aslight emitting elements, each of which has a single light emittingportion. The light beams discharged from the four light emittingelements are A1, A2, B1, and B2, respectively.

Referential numerals 111-114 stand for collimator lenses, which makevirtually parallel the light beams discharged from the light emittingelements 101-104. The collimator lenses 111-114 are positioned so thattheir axial lines coincide with the optical axes of the correspondinglight emitting elements.

Referential numerals 121 and 122 stand for half-wave plates, whichrotate 90 degrees the polarization directions of the virtuallyparallelized light beams A2 and B1.

Designated by referential numerals 131 and 132 are polarizing prisms, asa synthesizing means, which are synthesizing prisms of a certain type.The surface 131 a of the prism 131 transmits the entirety of the lightbeam A2, the polarization direction of which has been rotated 90degrees, while deflecting the entirety of the light beam A1, thepolarization direction of which has not been rotated. Therefore, thelight beams A1 and A2 are combined, without any loss, into a syntheticlight beam A, which is projected toward a half-mirror/synthesizing prism133. Similarly, the surface 132 a of the prism 132 allows the light beamB1 to transmit in its entirety the prism 132 while deflecting the lightbeam B2 in its entirety. As a result, the light beams B1 and B2 arecombined into a synthetic light beam B, which is projected toward thehalf-mirror/synthesizing prism 133.

In other words, the synthetic light beam A is the combination of thelight beams A1 and A2, which are different in the polarizationdirection, and the pitch between which in terms of the secondaryscanning direction has been set to a predetermined value. Similarly, thesynthetic light beam B is the combination of the light beams A2 and B2,which are different in the polarization direction, and the pitch betweenwhich in terms of the secondary scanning direction has been set to apredetermined value.

As for the half-mirror/synthesizing prism 133 (which hereinafter will bereferred to as half-mirror) as a synthesizing means, its surface 133 aallows one half of each of the light beams A and B to transmit throughthe half-mirror 133 while deflecting the other half of each of the lightbeams A and B. As a result, a light beam AB is synthesized with a lossof 50%. The light beam AB is circularly polarized by a quarter-waveplate 123, and is projected toward the cylindrical lens 3 shown in FIG.2. Instead of disposing a single quarter-wave plate 123 at the positionshown in FIG. 1, two quarter-wave plates 123 may be disposed between thepolarizing prism 131 and half-mirror 133 and between the polarizingprism 132 and half-mirror 133, one for one.

The synthetic light beam AB is the combination of light beams A1, A2,B1, and B2, the pitch among which in terms of the secondary scanningdirection has been set to a predetermined value.

In this embodiment, the light source apparatus 1 is structured so thatthe light beams A1 and A2 are allowed to pass through the polarizingprism 131 only once, and so that the light beams B1 and B2 are allowedto pass through the polarizing prism 132 only once.

Further, the light source apparatus 1 is structured so that the lightbeam A2 is allowed to pass in its entirely through all the synthesizingprisms (polarizing prism 131 and half-mirror 133) and constitutes theabsolute optical axis of the light source apparatus 1.

Next, referring to FIGS. 1, 4A, and 4B, the method, in accordance withthe present invention, for adjusting the pitch of the plurality of lightbeams will be described. FIGS. 4A and 4B are drawings depicting thelight beam pitch (physical scanning line interval) on the surface beingscanned, in terms of the secondary scanning direction.

If the values of the first and second light beam pitch setting, on thesurface being scanned, are 21.15 and 42.3 (μm) (1,200 and 600 dpi inresolution), respectively, the polarizing prism 132 is rotated by anangle adjusting means (not shown) about its lengthwise axis in thedirection w3, so that the pitch (physical scanning line interval)between the light beams B1 and B2 in terms of the secondary scanningdirection becomes 42.3 (μm) on the surface being scanned as shown inFIGS. 4A and 4B.

The pitch between the light beams B1 and B2 can also be adjusted to theaforementioned value by rotating the polarizing prism 132 in thedirection w4 about the optical axis of the light beam B1.

Further, the pitch between the light beams B1 and B2 can be adjusted tothe aforementioned value by rotating the light emitting element 103 or104 in the direction w5 about a horizontal axis perpendicular to theoptical axis of the light beam emitted from the light emitting element103 or 104, respectively, or moving the light emitting element 103 or104 in the vertical direction w6 perpendicular to the optical axis ofthe light beam emitted from the light emitting element 103 or 104,respectively.

Similarly, the pitch between the light beams A1 and A2 in terms of thesecondary scanning direction, on the surface being scanned, can beadjusted to 42.3 (μm), as shown in FIGS. 4A and 4B, by rotating thepolarizing prism 131, or rotating or moving the light emitting element101 or 102.

Next, the half-mirror 133 is rotated by an angle adjusting means (notshown), which is a switching means. Also rotated are the polarizingprisms 131 or 132, so that the pitch between the light beams A1 and B1(as well as the pitch between light beams A2 and B2) becomes 21.15 (μm)while keeping at 42.3 (μm) the pitches between the light beams A1 and A2and between the light beams B1 and B2. As a result, all the pitchesbetween two adjacent light beams, in terms of the secondary scanningdirection, become 21.15 (μm), as shown in FIG. 4A. In this case, theangle adjusting means constitutes the switching means.

Even if the order of the light beams A1 and B1 in terms of the secondaryscanning direction is changed, the pitches between two adjacent lightbeams become 21.15 (μm), as shown in FIG. 4B, realizing the first lightbeam pitch setting.

Further, the order of the light beams A1 and A2, and the order of thelight beams B1 and B2, in FIG. 4, do not need to exactly correspond tothose of the light beams A1 and A2 and the light beams B1 and B2, inFIG. 1. In other words, the positional switching between the light beamsA1 and A2 and between the light beams B1 and B2 does not create anyproblem.

Next, referring to FIGS. 4A, 4B, 5A, and 5B, the method, in accordancewith the present invention, for changing the light beam pitch in termsof the secondary scanning direction will be described. FIGS. 5A and 5Bare drawings depicting the light beam pitch, in terms of the secondaryscanning direction, on the surface being scanned.

It is as described above that the pitch between the light beams A and Bcan be varied by the rotation of the half-mirror 133. The opticalscanning apparatus can be switched from the first scanning setting, atwhich the pitch between any two adjacent light beams is 21.15 (μm), tothe second scanning setting, by rotating the half-mirror 133 by apredetermined angle from the position corresponding to the firstscanning setting.

More specifically, when the surface being scanned is being moved in thesecondary scanning direction and the light beam pitch of the lightsource apparatus 1 is as shown in FIG. 4A, the light beams B1 and B2 aremoved by 63.45 (μm) in the delaying direction (downward when lookingsquarely at the drawing) relative to the surface being scanned. As aresult, the pitch between the light beams A2 and B1 is changed to 42.3(μm) while the pitches between the light beams A1 and A2 and between thelight beams B1 and B2 are kept at 42.3 (μm), as shown in FIG. 5A.

Similarly, by moving the light beams B1 and B2, being projected as shownin FIG. 4A by 105.75 (μm) in the advancing direction (upward whensquarely looking at the drawing) relative to the surface being scanned,the pitch between the light beams B2 and A1 can be changed to 42.3 (μm),with the pitches between the light beams A1 and A2 and between the lightbeams B1 and B2 kept at 42.3 (μm), as shown in FIG. 5B.

It also is possible to change the light beam pitch as shown in FIG. 5Aor 5B, by moving the light beams B1 and B2, being projected as shown inFIG. 4B, either in the delaying direction (downward when squarelylooking at the drawing) by 105.75 (μm), or in the advancing direction(upward when squarely looking at the drawing) by 63.45 (μm).

Next, referring to FIGS. 2, 4A, 4B, 5A, and 5B, the means for switchingthe resolution of the optical scanning apparatus with the utilization ofthe light source apparatus 1 will be described.

When the light source apparatus 1 is set to the first scanning settingas shown in FIG. 4A or 4B, the light deflecting device 4 is rotated by amotor 13 in the direction w1 in the drawing at a constant velocity V,being controlled by a control system (not shown).

As the light source apparatus 1 is switched from the first scanningsetting to the second scanning setting shown in FIG. 5A or 5B, it isdetected by a detection circuit 72 that the light beams, which arescanning the peripheral surface of the photoconductive drum 6, havechanged in pitch.

The change in scanning setting can be detected as long as the lightsource apparatus 1 can be structured so that the pitch of each group oflight beams does not change with the elapse of time. For example, if thepitch between the light beams A1 and B1 is one half the pitch betweenthe light beams A1 and A2, the setting of the light source apparatus 1is the first position, whereas if the pitch between the light beams A2and B1 is equal to the pitch between the light beams A1 and A2, thescanning setting of the light source apparatus 1 is the second position.

However, when the pitch of each group of light beams varies with theelapse of time, the pitch of each group of light beams must beaccurately detected and must be adjusted, in addition to simplyswitching the scanning setting. Therefore, a structural arrangement ismade so that the optical distance from the light deflecting device 4 tothe light beam detecting surface of the detection circuit 72 via thedeflection mirror 71 becomes equal to the distance from the lightdeflecting device 4 to the peripheral surface f the photoconductive drum6.

When the detection circuit 72 is integrally disposed in the opticalscanning apparatus, it is sometimes difficult to make such a structuralarrangement that the aforementioned optical distance from the lightdeflecting device 4 to the light beam detecting surface becomes equal tothe distance from the light deflecting device 4 to the peripheralsurface of the photoconductive drum 6. In such a case, the ratio of thedistance from the light deflecting device 4 to the peripheral surface ofthe photoconductive drum 6, relative to the distance from the lightdeflecting device 4 to the light beam detecting surface, ispredetermined, and the positional relationship between the detectioncircuit and optical scanning apparatus is adjusted so that as theoptical scanning apparatus is attached to the main assembly of an imageforming apparatus or the like, the ratio of the distance from the lightdeflecting device 4 to the light beam detecting surface, relative to thedistance from the light deflecting device 4 to the peripheral surface ofthe photoconductive drum 6, becomes virtually equal to the predeterminedone.

The switching of the light beam pitch may be detected either by placingwithin the light source apparatus, a mechanism for detecting the angleby which the synthesizing prism is rotated, or by using the detectioncircuit 72 and this detecting mechanism in combination.

As the switching of the light beam pitch resulting from the change inthe resolution setting of an image forming apparatus is detected, thedetection signal is sent to a light deflecting device control system(controlling means) (not shown). As a result, the control system changesthe velocity of the light deflecting device 4 to V/2 by changing therevolution of the motor 13.

In addition, the detection signal is also sent to an image formationclock generation circuit (not shown). As a result, the clock generationcircuit changes the image formation clock frequency to ¼, changingthereby the resolution to ½ in terms of both the primary and secondarydirections.

The detection signal is also sent to an image formation control circuit(not shown). As a result, the image data with which the light beam B1 ismodulated, and the image data with which the light beam A1 is modulated,are switched by the image formation control circuit.

As described above, according to this embodiment of the presentinvention, the light beam pitch in terms of the secondary scanningdirection can be easily switched between two stages, that is, the firstand second scanning pitch settings; it can be switched by simplyrotating the half-mirror 133 with the use of the switching means.

When the scanning pitch setting of an optical scanning apparatuscomprising this light source apparatus 1 is switched to the secondposition, the light beam pitch in terms of the secondary scanningdirection can be seamlessly doubled by halving the defective surfaceswitching speed of the light deflecting device, making it possible toproject light beams so that the surface to be scanned can be scanned attwo different resolutions.

Further, in the case of a light source apparatus employing three or morelight emitting elements, the loss of the laser power can be minimized byemploying at least one synthesizing prism, as a synthesizing means, madeup of a polarizing prism.

Further, the light source apparatus is structured so that at least onelight beam from one of the light emitting elements is entirelytransmitted, making it possible to use this light beam as the absoluteoptical axis of the light source apparatus. Therefore, when mounting thelight source apparatus in the optical scanning apparatus, it is easy toaccurately position the two apparatuses relative to each other, andalso, to adjust the light beam pitch.

Embodiment 2

Next, referring to FIG. 6, the second embodiment of a light sourceapparatus in accordance with the present invention will be described.The components in FIG. 6, which do not have a referential numeral arethe same as those in FIG. 1. Therefore, they will be not be described.Further, the components which are identical in name to those in FIG. 1are identical in function to those with the same referential numeral.Therefore, they also will not be described.

FIG. 6 is a schematic perspective view of this embodiment of the lightsource apparatus. This light source apparatus is different from thefirst embodiment of the light source apparatus, and comprises four lightemitting elements, each of which has a single light emitting portion.

Designated by referential numerals 151 and 152 are half-mirrors assynthesizing means. The surface 151 a allows one half of each of thelight beams A1 and A2 to pass the half mirror 151 while deflecting theother half of each of the light beams A1 and A2. As a result, light beamA is synthesized at approximately 50% efficiency, and is projectedtoward the polarizing prism 153. Similarly, the surface 152 a allows onehalf of each of the light beams B1 and B2 to pass the half-mirror 152while deflecting the other half of each of the light beams B1 and B2. Asa result, a light beam B is synthesized by the half-mirror 152 at anefficiency of approximately 50%, and is projected toward the polarizingprism 153.

Designated by a referential numeral 161 is a half-wave plate, whichrotates 90 degrees the polarization direction of the light beam Aconverted into a virtually parallel luminous flux.

Designated by referential numeral 153 is a polarizing prism as asynthesizing means. It transmits the entirety of the light beam A, thepolarization direction of which has been rotated by 90 degrees by thesurface 153 a, while deflecting the entirety of the light beam B, thepolarization direction of which has not been rotated. Therefore, thelight beams A and B are combined, without any loss, into a syntheticlight beam AB. The synthetic light beam AB is circularly polarized by aquarter-wave plate 123, and is projected toward the cylindrical lens 3depicted in FIG. 2.

Also in this embodiment, the light source apparatus is structured sothat the light beams A1, A2, B1, and B2 are allowed to pass thepolarizing prism 153 only once.

Further, the light beam A2 is allowed to pass in entirety through allthe synthesizing prisms, that is, synthesizing prisms 151 and 153, beingused as the absolute optical axis of this light source apparatus.

In terms of the method for adjusting the light beam pitch and the methodfor switching the scanning pitch setting between the first and secondposition, this embodiment is the same as the first embodiment. In otherwords, the adjustment and switching are made by moving the synthesizingprism disposed at a position equivalent to the synthesizing prismposition in FIG. 1, in the same manner as the synthesizing prism in thefirst embodiment.

Embodiment 3

Next, referring to FIG. 7, the third embodiment of the light sourceapparatus in accordance with the present invention will be described.The components in FIG. 7, which do not have a referential numeral, areidentical to those in FIG. 1. Therefore, they will be not be described.Further, the components which are identical in name to those in FIG. 1are identical in function to those with the same referential numeral.Therefore, they also will not be described.

FIG. 7 is a schematic perspective view of this embodiment of the lightsource apparatus. This light source apparatus is another modification ofthe first embodiment of the light source apparatus, and comprises threelight emitting elements 801-803, each of which has a single lightemitting portion. Designated by referential numerals 811-813 arecollimator lenses.

Designated by referential numeral 831 is a polarizing prism. Thepolarizing prism synthesizes a light beam A by transmitting in entiretythe light beam A2, the polarization direction of which has been rotated90 degrees by a half-wave plate 821 while reflecting in entirety thelight beam A1, the polarization direction of which has not been rotated,and projects the light beam A toward a half-mirror 832.

The half mirror 832 is a synthesizing means, and synthesizes, at 50%efficiency, a light beam AB by transmitting one half of each of thelight beams A and B1. Then, it projects the light beam AB toward acylindrical lens similar to the cylindrical lens 3 shown in FIG. 2.

The light source apparatus in this embodiment is structured so that thelight beams A1 and A2 are allowed to pass the polarizing prism 831 onlyonce.

Further, the light beam A2 is allowed to pass in entirety through allthe synthesizing prisms, that is, synthesizing prisms 831 and 832, beingused as the absolute optical axis of this light source apparatus.

Effects similar to those described above can be obtained by replacingthe synthesizing prisms 831 and 832 with a half-mirror and a polarizingprism, respectively, and disposing the half-wave plate 821 at theposition contoured by a broken line in FIG. 7.

In such a case, the light beams A1, A2, and B1 are allowed to pass thepolarizing prism 832 only once.

Next, referring to FIGS. 7, 8A, and 8B, the method for switching thelight beam pitch in terms of the secondary scanning direction will bedescribed. FIGS. 8A and 8B are drawings depicting the light beam pitchon the surface being scanned, in terms of the secondary scanningdirection.

In order to switch the light beam pitch when the light beam pitchsetting is such that when it is at the first position, the light beampitch on the surface being scanned in terms of the secondary scanningdirection is 21.15 (μm) (1,200 dpi in resolution), whereas when it is atthe second position, the light beam pitch is 42.3 (μm) (600 dpi inresolution), the pitch between the light beams A1 and A2 (physicalscanning line interval) is adjusted to 42.3 (μm), as shown in FIGS. 8Aand 8B.

Next, the half-mirror 832 is rotated by an angle adjusting means (notshown) to adjust the pitch between the light beams A1 and B1 to 21.15(μm) while keeping the pitch between the light beams A1 and A2 at 42.3(μm), as shown in FIG. 8A. As a result, the pitch between any twoadjacent light beams in terms of the secondary scanning directionbecomes 21.15 (μm); in other words, the light beam pitch settingcorresponding to the first position is realized.

When the light beam pitch setting is as shown in FIG. 8A and the surfacebeing scanned is moving in the secondary scanning direction indicated byan arrow mark also as shown in FIG. 8A, the second light beam pitchsetting, at which the pitch of any two adjacent light beams is 42.3(μm), as shown in FIG. 8B, can be realized by moving the light beam B1by 63.45 (μm) either in the delaying direction (downward when squarelylooking at the drawing), or the advancing direction (upward whensquarely looking at the drawing), relative to the surface being scanned.

Embodiment 4

Next, referring to FIG. 9, the fourth embodiment of the light sourceapparatus in accordance with the present invention will be described.The components in FIG. 7, which do not have a referential numeral, areidentical to those without a referential numeral in FIG. 1. Therefore,they will be not be described. Further, the components which areidentical in name to those in FIG. 1 are identical in function to thosewith the same referential numeral. Therefore, they also will not bedescribed.

FIG. 9 is a schematic perspective view of the light source apparatus inthis embodiment. This light source apparatus is another modification ofthe first embodiment of the light source apparatus in accordance withthe present invention, and comprises five light emitting elements901-905, each of which has a single light emitting portion. Designatedby referential numerals 911-915 are collimator lenses.

Designated by referential numeral 931 is a half-mirror as a polarizingmeans. The half-mirror 931 synthesizes, at 50% efficiency, a light beamA′ by transmitting one half of each of the light beams A1 and A2 whilereflecting the other half of each of the light beams A1 and A2, andprojects the light beam A′ toward a half-mirror 932.

Similarly, a half-mirror 932 as a synthesizing means synthesizes a lightbeam A from the light beams A′ and A3, and projects the light beam Atoward a polarizing prism 934. Further, a half-mirror 933 as asynthesizing means synthesizes a light beam B from light beams B1 andB2, and projects the light beam B toward a polarizing prism 934.

The polarizing prism 934 as a synthesizing means synthesizes a lightbeam AB by transmitting in entirety the light beam A, the polarizationdirection of which has been rotated 90 degrees by a half-wave plate 921,while reflecting in entirety the light beam B, the polarizationdirection of which has not been rotated. Then, it projects the lightbeam AB toward a cylindrical lens identical to the cylindrical lens 3 inFIG. 2, after circularly polarizing the light beam AB by a quarter-waveplate 922.

Next, referring to FIGS. 9, 10A, and 10B, the method for switching thelight beam pitch in terms of the secondary scanning direction will bedescribed. FIGS. 10A and 10B are drawings depicting the light beam pitchon the surface being scanned, in terms of the secondary scanningdirection.

It is assumed that the light beam pitch setting is such that when it isat the first position, the light beam pitch on the surface being scannedin terms of the secondary scanning direction is 21.15 (μm) (1,200 dpi inresolution), whereas when it is at the second position, the light beampitch is 42.3 (μm) (600 dpi in resolution). In order to switch the lightbeam pitch from the first light beam pitch setting to the second lightbeam pitch setting, the pitch among the light beams A1, A2, and A3(physical scanning line intervals) is adjusted to 42.3 (μm), as shown inFIGS. 10A and 10B.

Similarly, the pitch between the light beams B1 and B2 (scanning lineintervals in physical terms) is adjusted to 42.3 (μm).

During this adjustment, the light beams B1 and B2 may be switched inposition. Further, the positional order among the light beams A1, A2,and A3 does not need to be as it is presented here.

Next, the polarizing prism 934 is rotated by an angle adjusting means(not shown) to adjust the pitch between the light beams A1 and B1 to21.15 (μm) while keeping the pitch among the light beams A1, A2, and A3,and the pitch between the light beams B1 and B2, at 42.3 (μm), as shownin FIG. 10A. As a result, the pitch between any two adjacent light beamsin terms of the secondary scanning direction becomes 21.15 (μm); inother words, the light beam pitch setting corresponding to the firstposition is realized.

When the light beam pitch setting is as shown in FIG. 10A and thesurface being scanned is moving in the secondary scanning directionindicated by an arrow mark also in FIG. 10A, the second light beam pitchsetting, at which the pitch of any two adjacent light beams is 42.3(μm), as shown in FIG. 10B, can be realized by moving the light beams B1and B2 by 105.75 (μm) either in the delaying direction (downward whensquarely looking at the drawing), or in the advancing direction (upwardwhen squarely looking at the drawing), relative to the surface beingscanned.

Embodiment 5

Effects similar to those obtained by the above described first to fourthembodiments can also be obtained by replacing the light source portioncomprising the plurality of light emitting elements, each of which has asingle light emitting portion, and a half-mirror as a synthesizingportion, with a single light emitting element having a plurality oflight emitting portions.

Next, referring to FIGS. 11A and 11B, an example of a light sourceportion in the fifth embodiment of the light source apparatus inaccordance with the present invention will be described.

FIG. 11A shows the portion of this embodiment of the light sourceapparatus equivalent to the portion of the light source apparatus inFIG. 9, from the light emitting elements 901 and 902 to the half-mirror931 as a synthesizing means for combining the light beams A1 and A2.FIG. 11B shows a modification of the structure shown in FIG. 11A, inwhich the components shown in FIG. 11A have been replaced with thecombination of a single light emitting element 901′ having two lightemitting portions and a collimator lens 911′. Referring to FIG. 11B, thelight beams A1 and A2 emitted from the light emitting element 901′ aremade virtually parallel by the collimator lens 911′ disposed in such amanner that its axis coincides with the optical axis of the light beamA′ (A1, A2).

The pitch between the two light beams emitted, one for one, from the twolight emitting portions of the single light emitting element becomesfixed when the light emitting element is manufactured. Therefore, theadjustment is made by rotating the light emitting element about itsoptical axis.

As described above, according to the present invention, a light sourceapparatus is structured so that a plurality of light beams are dividedinto two groups of light beams, which are fixed in light beam pitchwhile being different in light beam pitch. Further, it is provided witha switching means for switching the positional relationship between thetwo groups of light beams, in terms of the secondary scanning direction,on the surface being scanned. Therefore, the light beam pitch of thelight source apparatus can be easily switched between two settings.

Further, at least one of the synthesizing means is made up of apolarizing prism, and all the light beams are allowed to pass thepolarizing prism no more than once. Therefore, the laser efficiency lossis minimized (power is reduced to half).

Further, at least one of the light beams is allowed to pass in entiretythe optical components, making it possible to use it as the absoluteoptical axis of the light source apparatus. Therefore, the adjustment ofthe light beam pitch in terms of the secondary scanning direction, andthe resolution switch, can be easily made.

While the invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth, and thisapplication is intended to cover such modifications or changes as maycome within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. An optical scanning apparatus for scanning arotating photosensitive member with a light beam deflected by arotatable polygonal mirror, said optical scanning apparatus comprising:a first light source for outputting a first light beam; a second lightsource for outputting a second light beam; a third light source foroutputting a third light beam; a first optical unit for adjusting adistance between imaging positions of the first light beam and thesecond light beam to a predetermined distance with respect to arotational moving direction of the photosensitive member, wherein saidfirst optical unit is rotatable to adjust the imaging positions of thefirst light beam and the second light beam; and a second optical unitfor adjusting an imaging position of the third light beam relative tothe first light beam and the second light beam, the distance betweenwhich is adjusted to the predetermined distance, with respect to therotational moving direction of the photosensitive member, wherein saidsecond optical unit is rotatable to adjust the imaging position of thethird light beam relative to the imaging positions of the first lightbeam and the second light beam, wherein said second optical unit isrotatable independently of said first optical unit, and wherein byrotational adjustment of said second optical unit, the third light beamcan be imaged between the first light beam and the second light beamwith respect to the rotational moving direction of the photosensitivemember, or the first light beam and the third light beam can be imagedat positions across the second light beam from each other with respectto the rotational moving direction of the photosensitive member.
 2. Anapparatus according to claim 1 wherein when the third light beam isimaged between the first light beam and the second light beam, adistance between the imaging positions of the first light beam and thethird light beam and a distance between the imaging positions of thethird light beam and the second light beam can be adjusted to be onehalf of the predetermined distance, and when the first light beam andthe third light beam are imaged at positions across the second lightbeam from each other with respect to the rotational moving direction ofthe photosensitive member, a distance between the imaging positions ofthe second light beam and the third light beam can be adjusted to be thepredetermined distance.